The Ethernet physical layer is an example of a high frequency communications network physical layer in which the physical medium over which one or more electrical signals can be communicated can include an Ethernet cable with one or more communication lines that can include one or more respective twisted pairs of wires. Ethernet communication can encode data as differential signals on these communication lines for high frequency communication, for example, at 125 MHz. Full duplex data communication over the physical medium means that an incoming received signal may be present on a pair of wires at the same time that an outgoing transmitted signal is present on that same pair of wires. Parasitic capacitance and other non-idealities of the Ethernet cable can result in the differential data signal being accompanied or overwhelmed by a common-mode signal.
At a particular communications network node, a received digital signal should be quickly amplified from a small differential signal present on the pair of wires to a larger differential signal that can provide suitable signal levels for differentially or single-endedly driving logic or other circuits within a receiver circuit at that particular network node. Such small differential signals present on the pair of wires should be detected even in the presence of a common mode signal that may be present on the pair of wires, such as may be due to circuit or environmental noise or differences between receiving and transmitting locations.
To achieve both differential signal amplification and common mode signal attenuation, a center tap auto-transformer balancing circuit can be used at an Ethernet network node, such to provide a high input impedance for the differential data signal, for its amplification, and a low input impedance for the common mode signal, for its attenuation.